Stable Isotope Paleodiet & Mobility Analysis

Buried bone collagen can degrade and exchange carbon and nitrogen with the soil, shifting its isotope values away from the original dietary signal. Well-preserved collagen has a carbon-to-nitrogen atomic ratio in a narrow range (about 2.9 to 3.6) along with adequate collagen yield and sensible elemental percentages; values outside these bounds signal contamination or breakdown. Screening on these criteria, as Bentley and the wider literature insist, ensures that the carbon and nitrogen values reflect what the person ate rather than what happened to the bone in the ground.

Nitrogen-15 becomes enriched by roughly three to five per mil at each step up the food chain, so the gap between a human's nitrogen value and the local herbivore baseline estimates how many trophic levels of animal protein the diet contained. This gives a continuous measure of carnivory rather than a yes/no answer. Its limits are that the per-step enrichment varies somewhat among species and environments, that aquatic food webs add extra steps which can mimic high meat intake, and that the inference requires a properly measured local baseline; carbon data are used alongside nitrogen to disentangle these possibilities.

Strontium enters teeth through the food chain, not directly from rock, and the strontium that plants and animals actually take up — the biologically available fraction — can differ substantially from the bulk bedrock ratio because of weathering, atmospheric input, and mixing of sources. Comparing a person's enamel to the bedrock value can therefore misclassify locals as migrants or vice versa. Bentley's review stresses estimating the bioavailable range from modern plants, water, snails, or archaeological fauna so that 'local' is defined by what was actually eaten, making mobility inferences valid.